1. Field of the Invention
The present invention relates to a process for the preparation of catalysts for the polymerisation of olefins, to the catalysts obtained by this process and to a process for polymerising olefins in the presence of these catalysts.
2. Description of the Related Art
It is known to polymerise olefins such as ethylene by means of a catalyst containing chromium deposited on a support which may be silica, alumina, zirconia or thoria. The synthesis of these catalysts in most cases comprises a final stage of calcination in an oxidising atmosphere, under conditions such that part of the chromium is converted to hexavalent chromium (U.S. Pat. No. 2,825,721 (Phillips Petroleum)).
These catalysts are generally obtained by dispersing the support in aqueous or organic solutions of chromium compounds. The industrial production of catalysts according to this process involves drying the catalysts, recycling the chromium solutions and treating effluents liable to contain chromium compounds before they are discharged. Such a process is long and complex.
A simpler process consists in performing an intimate and solvent-free mixing of chromium trioxide (CrO.sub.3) and of the support and then calcining it (U.S. Pat. No. 2,825,721 (Phillips Petroleum)). The dispersion of the chromium on the support is improved by performing the mixing of the reactants at a temperature of between 93 and 190.degree. C. (Patent GB-814,295 (Phillips Petroleum)). The catalysts thus obtained nevertheless have a large quantity of chromium sesquioxide (Cr.sub.2 O.sub.3) agglomerates which are inactive in polymerisation, which are found again, unchanged, in the polymers whose properties they alter.
The preparation of catalysts containing chromium by dry mixing of the supports with organic chromium compounds, followed by a heat treatment under a nonoxidising atmosphere comprising successive plateaus at 177.degree. C., 290.degree. C. and 900.degree. C. has also been described (U.S. Pat. No. 3,953,413 (Chemplex Co.)). The catalysts obtained by this process contain black carbon residues and exhibit mediocre activities. Their performance can be improved if the heat treatment is terminated by a posttreatment in an oxidising medium for a sufficiently short period to remove the carbon residues (U.S. Pat. No. 4,053,437 (Chemplex Co.)). Such a particularly long and complex procedure involves the use of sophisticated pieces of apparatus capable of working in an inert atmosphere.